Repair and service works at a ship's hull are usually performed in a floating dock. This should have the capacity to lift the ship, possibly containing some cargo, and includes a bottom member dimensioned with respect to the biggest ship expected to be handled, and two longitudinal side walls.
The side walls form a considerable obstacle during work in the dock, they make transports to and from the working sites below the hull more difficult, and they prevent the use of rational machines for instance for cleaning and painting the bottom plating.